Iowa / Mahaska County
Score 47 · elevated · #48 of 99 in IA
Crawl space moisture in Mahaska County, IA
Mahaska County scores elevated because several moderate signals stack up rather than one dominant one. Where a seasonal water table is reported — 91% of acres — it averages 2 ft below grade.
- 1Water table 2 ft
- 2Wet soils 33%
- 3Moisture score 47
- 4EPA radon zone 1
Reading the Mahaska County soil survey
The score of 47 combines four parts, each from the USDA survey: wet drainage classes contribute 47.1 points of 100 on their own scale, group D soils 44.4, a shallow water table 62.7, and flooding 10.5; the weights are on the methodology page. The dominant hydrologic group is C (55% of acres): slow infiltration (clay layers).
At 2 ft the seasonal water table is usually below a crawl-space floor, but the 91% of acres that report one are by definition the wettest parts of the county; on those lots, a wet spring can bring water close to the footings.
Mahaska County against the rest of Iowa
Mahaska County ranks 48 of 99 Iowa counties, level with the state median of 47. Wetter counties exist in Iowa, but the soil here still sits well above the national middle. Statewide, 44% of mapped soil is in the wet drainage classes; here it is 33%, below the state figure — the score here comes from a shallow water table rather than drainage class alone. After moderately well drained, the next most common class is poorly drained at 19%.
Flooding. 2.1% of acres with a flooding class are rated frequent; 13.7% of acres with a flooding class are rated occasional; 1% of acres with a flooding class are rated rare. That is a large flood-prone share; a crawl space in the floodplain needs flood vents under most local floodplain ordinances, which is a separate rule from R408 and can rule out a fully sealed design.
What it means for a crawl space here
In an elevated county the soil supplies more moisture than average, but a vented crawl space can work if the ground is covered: the code lets vent area drop from 1 sq ft per 150 to 1 per 1500 with a Class I vapor retarder on the ground, which is an admission of how much moisture comes from below. Grading, gutters and a ground cover usually decide whether a crawl space here stays dry before any decision about sealing it.
Radon. Mahaska County is in EPA radon zone 1, the highest predicted potential. A sealed crawl space and a radon system share most of their parts — the liner, the sealed seams, a fan — so if you are encapsulating, test for radon first and design both together. EPA itself says the zone map is not a reason to skip testing in zones 2 and 3.
Near Mahaska County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey area IA123; drainage covers 99% of 365,800 mapped acres); EPA Map of Radon Zones, county table (June 2024 JSON); IRC R408.3 Unvented crawl space, as adopted in S.C. Code Regs. 8-1221 (LII). Checked 2026-10-23.